【ANDROID STUDIO】 Fragment Back Stack

package com.example.fragmentbackstack;

import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.Fragment;
import androidx.fragment.app.FragmentManager;
import androidx.fragment.app.FragmentTransaction;

import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity
        implements FragmentManager.OnBackStackChangedListener {

    Button mButtonNext;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        getSupportFragmentManager().addOnBackStackChangedListener(this);

        mButtonNext = findViewById(R.id.buttonNext);
        mButtonNext.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                FragmentManager fragmentManager = getSupportFragmentManager();
                FragmentTransaction fragmentTransaction =
                        fragmentManager.beginTransaction();
                fragmentTransaction.replace(R.id.frameLayout, new FragmentTwo());
                fragmentTransaction.addToBackStack(null);
                fragmentTransaction.commit();
                mButtonNext.setVisibility(View.INVISIBLE);
            }
        });

        FragmentManager fragmentManager = getSupportFragmentManager();
        FragmentTransaction fragmentTransaction =
                fragmentManager.beginTransaction();
        fragmentTransaction.add(R.id.frameLayout, new FragmentOne());
        fragmentTransaction.addToBackStack(null);
        fragmentTransaction.commit();
    }

    @Override
    public void onBackPressed() {
        if (getSupportFragmentManager().getBackStackEntryCount() == 2) {
            super.onBackPressed();
            mButtonNext.setVisibility(View.VISIBLE);
        } else {
            finish();
        }
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem menuItem) {
        if (menuItem.getItemId() == android.R.id.home) {
            onBackPressed();
            return true;
        } else {
            return super.onOptionsItemSelected(menuItem);
        }
    }

    @Override
    public void onBackStackChanged() {
        Fragment fragment = getSupportFragmentManager().findFragmentById(R.id.frameLayout);
        if (fragment instanceof FragmentOne) {
            getSupportActionBar().setDisplayHomeAsUpEnabled(false);
        } else if (fragment instanceof FragmentTwo) {
            getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        }
    }
}

package com.example.fragmentbackstack;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.fragment.app.Fragment;

public class FragmentOne extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_one,
                container, false);
    }
}

package com.example.fragmentbackstack;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import androidx.fragment.app.Fragment;

public class FragmentTwo extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        return inflater.inflate(R.layout.fragment_two,
                container, false);
    }
}
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <FrameLayout
        android:id="@+id/frameLayout"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_above="@+id/buttonNext"
        android:layout_alignParentTop="true"></FrameLayout>

    <Button
        android:id="@+id/buttonNext"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerInParent="true"
        android:text="Next" />
</RelativeLayout>
fragment_one.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="match_parent"
    android:layout_width="match_parent">
    <TextView android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Fragment One"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />
    </RelativeLayout>
fragment_two.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_height="match_parent"
    android:layout_width="match_parent">
    <TextView android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Fragment Two"
    android:layout_centerVertical="true"
    android:layout_centerHorizontal="true" />
    </RelativeLayout>